Power Issue with Duemilanove

Hi,

I am a first year Mechanical Engineering student at the Florida Institute of Technology. I have a project in my Intro class where my team and i have to create an autonomous robot to compete in a demolition derby. It is nearly complete, but the only problem is i am not sure how to power the weapon. We have two diamond edge saw blades on either side of our vehicle that are powered by a central motor that drives them via a pulley system. Now here is the problem: The motor draws a steady state current of 1.3A but when hit can get as high as 3-4A. We are restricted to using 8 AA NiMH batteries.

Now if i wire these batteries in parallel, the motor will have plenty to work with b/c we get 3.4Ah instead of 1.7Ah. But under this configuration the voltage will be b/t 4.8-5.4V. The issue here is powering the arduino. If i run the batteries in series, then the arduino is fine, but the motor is limited greatly.

Now keep in mind i also have a servo and a motor to power which draw a maximum of 400mA and 500mA, respectively. Now this isn't that big of an issue b/c the servo will not be on all the time and with PWM i can run the drive motor at whatever speed i want. Also keep in mind this vehicle only has to run for 3minutes.

Now a friend of mine who is an EE major suggested a 5V DC to DC Step up Vpack PCB, but he said the only problem is that it is limited to 300mA. So the board can't draw more than that from this chip. I'm not quite sure how to solve this issue. Will this chip work? What other alternatives are there? I have until next Tuesday to get this done.

Thanks, John.

Link for the DC Step up: 5V DC to DC Step Up - VPack PCB - PRT-08290 - SparkFun Electronics

Ah is a rating for how much power a battery can deliver for how long. It is a capacity rating. Series or parallel arrangements can not change the capacity of a battery.

The Arduino should not be powering any motors or servos. All it should be doing is controlling them - turning them off or on at appropriate times.

The Arduino itself uses very little current, so the 300mA provided by the Vpack will be plenty.

I have until next Tuesday to get this done.

Good thing you started early.

A couple of details are missing, like what the saw motor voltages are supposed to be. What was their voltage when they were drawing 1.3A?

Also, the amp-hour rating of the battery is not the same as the kind of current (amps) it can deliver. Amp-hours is a measure of energy storage capacity, not current delivery.

If you try to draw 3-4A from your battery pack you will see the voltage drop significantly.

--
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

Below is a UBEC power converter that is very efficient for dropping voltage. Bottom is how I protect a servo control chip from power low voltage dropouts due to high motor starting loads using a diode and capacitor .

How about:

+--[ bat1 ]--[ bat2 ]--[ bat3 ]--+--[ bat 7 ]--[ bat 8]--+
+-+ | |
| +--[ bat4 ]--[ bat5 ]--[ bat6 ] + v Vin
| |
v Gnd v Vmot

Gives you 4V for the motors and 7V to feed the Arduino.

Alright...a few clarifications. The project is due tuesday, but we have been working on it for a month...so no, my team are not procrastinating.

I understand what amp-hours are. I thought that if i run two packs of four batteries in parallel i would get 3.4 Ah to work with instead of 1.7Ah(rating for one battery). I understand that this does not mean that the batteries will provide 3.4 constantly, but that they can provide 3.4 for an hour.

The saw motor is rated for 3-9V. I have been giving it 4.8-5.4V (four batteries). The problem is that want to run the saw motor at full blast without running the batteries down too fast. let me know if this is correct:

If i am running all 8 batteries in series, that means i have about 9.6V and 1.7Ah. If i want a constant current of 3A (probably higher than actual current draw) that means:

1.7A1h=3Axh
x=.57h=34min.

So does this mean i will be safe running the saw motor at full blast while still moving for 3min (derby round length). Or is there something i am not considering?

Thanks, John.

I thought that if i run two packs of four batteries in parallel i would get 3.4 Ah to work with instead of 1.7Ah(rating for one battery).

The capacity of the batteries doesn't change when you use them in a serial configuration, a parallel configuration, a star configuration, etc. They can provide what they can provide, and then they are dead. :frowning:

The problem is that want to run the saw motor at full blast without running the batteries down too fast.

If I had limited capacity like that, I'd dedicate two batteries for defense (i.e. to run away) and 6 for offense (i.e. the saw blade). When the saw blade is near nothing, running it all full speed is a waste of energy. Figuring out if there were something nearby to cut, and ramping the saw up to full speed as the object got closer would be a better use of available energy.

Assuming your batterys are rechargable, why not try hooking up both ways and get some real world data. Real world performance will be more accurate and quicker than chit-chat.